The invention generally relates to a projection system.
Referring to FIG. 1, a typical mirror-based projection system 20 may include at least one mirror array 10 (a mirror array of a digital micromirror device (DMD), for example) that reflects light in a manner that produces an image on a projection screen 19. The mirror array 10 includes mirrors that are selectively tilted to spatially control the reflection of light (from a light source (not shown)) to and away from the screen 19 to form the image. More specifically, in the projection system 20, each mirror of the array may be uniquely associated with one pixel of the image so that the mirror controls the intensity of the associated pixel. The projection system 20 controls the tilt angle of each mirror to control when the mirror reflects light into a projection cone 18 of light that projection optics 16 (of the system 20) casts onto the projection screen 19. Depending on the desired pixel intensity, the projection system 20 may tilt a particular mirror at a first angle to reflect light into the cone 18 of light to illuminate the associated pixel, or the projection system 20 may tilt the mirror at another angle to reflect light away from the cone 18 of light and darken the associated pixel.
As a more specific example, an exemplary state of the mirror array 10 is depicted in FIG. 2. Assuming a two tone black and white projected image for this example, some mirrors (such as the mirrors 12) of the array 10 are associated with black pixels of the image, and other mirrors (such as mirrors 13) of the array 10 are associated with white pixels of the image. The mirrors that are associated with the black pixels are tilted at angles to reflect incident light away from the cone 18 (FIG. 1) of light; and the mirrors that are associated with white pixels are tilted at angles to reflect the incident light into the cone 18 of light. The reflection of light into or away from the cone 18 of light is depicted in FIG. 1 for an exemplary mirror 14 of the array 10. The mirror 14 controls the intensity of an associated pixel 11 of an image that is formed on the projection screen 19. To produce a white pixel, the projection system 20 tilts the mirror 14 at an angle to reflect light along a path 17 that intersects the pixel 11 and is within the cone 18. To produce a black pixel, the projection system 20 tilts the mirror 14 at an angle to reflect light along a path 15 that falls outside of the cone 18.
To create intermediate pixel intensities (called gray scale intensities) other than the two tone intensities described above, the projection system may use pulse width modulation (PWM). With PWM, the projection system controls the tilt angle of each mirror pursuant to a PWM cycle to establish a particular intensity value for a given pixel. More specifically, pursuant to a PWM cycle, a gray scale intensity for a particular pixel is created by moving the associated mirror rapidly between a position in which the mirror reflects light into the cone 18 (during an “on time” of the PWM cycle) and a position in which the mirror directs the incident light along a path outside of the cone 18 (during an “off time” of the PWM cycle). The fraction of time in which light is directed toward the pixel as compared to the duration of the PWM cycle determines the average brightness, or gray scale intensity, of the pixel. The viewer's eyes integrate these rapid flashes into a perception of a gray scale intensity for the pixel. The gray scale in this sense also applies to color images that result from a projection system that combines red, green and blue images (created by directing red, green and blue light beams toward the mirror array 10) to form a color image.
The conventional projection system updates the projected image pursuant to a frame rate (a 60 Hz rate, for example). This frame rate places a limit on the duration of the PWM cycle, as the PWM period (the inverse of the frame rate) cannot extend beyond the frame period. Other factors may compress the time that is allocated for each PWM cycle. For example, less time is allocated to each PWM cycle if the number of rows in the projected image exceeds the number of rows of mirrors in the mirror array. For this arrangement, the projection system may generate a perceived projected image by scanning the perceived image (once every frame period) with the images that are created by the mirror array. This means multiple updates must be made to the mirror array during each frame period, thereby decreasing the time allocated for each PWM cycle by the corresponding multiple. Furthermore, the mirror array may be used to sequentially form three primary color (red, green and blue, for example) images to produce a perceived color composite image, thereby further compressing the time allocated to each PWM cycle. Additionally, the time allocated to each PWM cycle may be further reduced by multiple refresh operations that redraw the image that the user sees on the projection screen 19 several times during each frame period. Thus, a significantly small time may be allocated for the PWM cycle so that a potential challenge associated with the above-described approach is that the mirror array may not be capable of moving its mirrors rapidly enough to accommodate the allocated PWM cycle time.
Thus, there is a continuing need for an arrangement and/or technique that addresses one or more of the problems stated above as well as an arrangement and/or technique that addresses one or more problems that may not be set forth above.